The need to achieve adequate thermal distribution within an optical element such as a laser rod, necessitates encompassing the laser rod with a thermally conductive material, such as ceramic or sapphire. The thermally conductive material may be of cylindrical geometry, completely encompassing the rod or in the form of a hemisphere, where only half the rod is encompassed. The thermally conductive material is bonded to the rod by an optical adhesive which provides optical matching between the thermally conductive material and the rod. The adhesive also serves to thermally connect the rod and the thermally conductive material.
A problem arises, however, when the rod assembly undergoes variations in temperature; such variations cause the dissimilar materials of the optical element, thermally conductive material and optical adhesive to expand at different rates. When the temperature rises, high stresses are produced in the assembly which can cause the adhesive to fail, resulting in air pockets between the rod and the thermally conductive material. The air pockets result in an optical interference between the rod and the pump source. Moreover, the air pockets cause an increase in thermal resistance between the rod and the thermally conductive material, causing increased rod temperature. In addition, the stresses produced may result in the development of birefringence in the optical element, causing a change in the polarization vector of the beam. Furthermore, the stresses may be high enough to cause the eventual fracture of the optical element or thermally conductive material.